Electrical capacitors made of thin film dielectric material which is metallized on both sides are used in large quantities in electrical devices because of their reliability, their desirable electrical characteristics and their relatively low cost. U.S. Pat. No. 2,470,826 issued on May 24, 1949 to W. McMahon illustrates a capacitor in which a double sides metallized dielectric layer is folded once in a longitudinal direction. The folded dielectric material may then be wound along its longitudinal dimension and electrical connections are made to the single pleated metallized dielectric layer of the McMahon patent by leads that are wound into the capacitor. The type of capacitor described by McMahon patent is extremely desirable because substantially all of the dielectric layer of the capacitor is in the electric field and the capacitor may be made without the insertion of an additional dielectric layer in the main body of the capacitor, although a short initial unmetallized dielectric strip and a protective terminating dielectric strip may be employed with the capacitor, if desired.
The type of capacitor construction that is envisioned by McMahon patent has a number of decided manufacturing and electrical advantages. For example, the problem of masking the dielectric layer during electrode evaporation when the electrode areas are applied to the film is substantially reduced. In addition, no slitting is required as it is during the manufacture of conventional wound film capacitors. Furthermore, the capacitance per unit dielectric area is maximized because of the substantially full use of the dielectric layer in the electric field, and, thus, shorter winding lengths per unit capacitance are required, thereby reducing the winding labor. Additionally, the capacitance lengths can be calibrated and pre-cut to produce a capacitor of more accurate values. Another advantage of this type of capacitor is that there are no air layers between the metallized electrodes and the pleated film surfaces and this increases the voltage at which corona discharge starts.
The McMahon patent suggested that leads could be attached to his pleated and wound capacitor by the well-known Schoop process when at least three pleats were employed. Actually, however, the application of high velocity metallic spray coatings to the edges of the capacitor described by McMahon is not commercially practical because metallic spray will penetrate through the dielectric area at a pleat, thereby shorting one electrode to the other. If the temperature and pressure of the spray are lowered, penetration of the dielectric layers may be reduced but the adhesion of the spray will generally be very poor. The type of multiple leads that were suggested in the McMahon patent that were wound into the capacitor also is not satisfactory since these leads tend to tear the dielectric material and they make the capacitor bulky and inductive.
In order to solve the above mentioned shorting problem at a pleated edge, it has been suggested in Australian Pat. No. 159,958, patented on Nov. 24, 1954 to provide unmetallized dielectric strips behind all of the pleats of a pleated metallized capacitor. The employment of an unmetallized strip behind the single pleated capacitor of the McMahon patent, however, still would not provide an end termination along the opposite edge of the wound capacitor segment that would allow a metallized spray to be applied to this edge because the ends of the dielectric layer of the McMahon patent at this edge extended beyond both of the metallized electrodes and contact between the metallic spray and the inner electrode area would thereby be prevented.
The configuration of the capacitor of the present invention provides a product which has all of the advantages of single pleated, or single pleated and wound, capacitors and also has the added advantage that it may be manufactured with commercially available metallized dielectric material. A further advantage of the present invention is that the same basic starting material may be used to manufacture capacitors having a number of different pleating combinations, which, of course, is not true for the capacitor of the Australian patent, in which each pleat combination requires a specially manufactured dielectric layer. In addition to the use of the present invention to manufacture wound film capacitors, it is also possible to utilize the invention to produce ceramic capacitors in which the capacitor dielectric material is folded while the ceramic material is still in a "green" state, in a manner similar to that disclosed in U.S. Pat. No. 3,223,494 issued Dec. 14, 1965 to J. W. Crownover.